Television receiver



Jan. 6, 1942. o. L. PLAISTOWE 2,268,671

' TELEVISION RECEIVER Filed July 16, 1938 INVENTOR DONALD L. PLAISTOWE AT I'ORNEYS 5 7 7 SYNC. i aurpur Patented Jan. 6, 1942 TELEVISION RECEIVER Donald Leopold Plaistowe, Chelmsford, England, assignor to Radio Corporation of America, a

corporation of Delaware Application July 16, 1938, Serial No. 219,495 In Great Britain August '7, 1937 2 Claims.

This invention relates to television receivers and more specifically to television receivers of the kind adapted for use in cooperation with transmitters which transmit picture signals and synchronizing signals on the same carrier wave, the synchronizing signals being separated out from the picture signals at the receiver for use to synchronize the said receiver.

In practice the maintenance of correct synchronism of a receiver of the kind referred to, presents serious difficulties, more especially where the receiver is situated at a point near the extreme range of a transmitter or at some other point where the noise to signal ratio is high. It is found that random surges, pulses and disturbances generally will interfere with synchronism sufiiciently to cause distortion of the raster or scanning pattern and may often cause complete loss of synchronism. This type of difficulty is accentuated by reason of the fact that, owing to the wide range of modulating frequencies involved in high fidelity television, the pass range of the signal stages of a television receiver is necessarily high-plus or minus 2 megacycles in a typical modern pass range-so that there is great liability to noise and disturbances. The present invention seeks toavoid these difficulties.

According to this invention a television receiver of the kind referred to comprises a synchronizing signal separating circuit arrangement including an electron beam valve having its control electrode circuit coupled to a point in the signal channel of the receiver, and its output electrode coupled through a. band pass circuit whose band width is very small relative to the television signal band width to a synchronizing signal selector circuit as known per so. By reason of the use of an electron beam valve in the manner described a very high degree of freedom from interaction between the band-pass circuit and the television signal circuits of the receiver is obtained, while the noise level is decreased and the synchronizing signal level increased as regards energy fed to the synchronizing signal selector circuit.

The invention is illustrated in the accompanying drawing wherein the single figure shows diagrammatically one way of carrying out the invention the said drawing showing only those parts of a receiver necessary to an understanding of the preesnt invention.

Referring to the drawing the anode I of a valve 2 in the intermediate frequency portion of a television receiver of the superheterodyne type-the band pass range of this intermediate frequency amplifier may, in a typical case, be plus or minus 2 megacyclesis capacity coupled by means of a condenser 3 to the control electrode 4 of an electron beam valve 5. The control electrode 4 is connected to the cathode 6 of the valve 5 through a resistance 1. The valve 5 is shown as of the known construction having, in addition to the cathode 6 and the control electrode 4, an apertured screen electrode 8, a grid like screen electrode 9-, and a final collector electrode or anode Ill. The anode I0 is connected through a resistance ll and source of potential 12 to the cathode, the two screens 8, 9 being biased from suitable points on said source as in the usual way. The anode I0 is also capacity coupled by a condenser l3 to the input terminal of a band pass filter M of narrow pass range. This range is preferably not more than plus or minus k. c. and is preferably only about plus or minus 50 k. c. For example the band pass filter may consist of two inductively coupled parallel tuned circuits I4a, Mb, connected together and to the cathode 6 of the electron beam valve 5 at one end, the remaining end of one tuned circuit being connected through condenser l3 to the anode l0, and the remaining end of the other tuned circuit being coupled to the grid (not shown) of a further valve (not shown) adjusted and arranged in any convenient manner to select the synchronizing signals from the narrow band passed. In some cases this selector valve will operate in the grid current region and in such a case the present invention offers the advantage that this valve despite working with grid current does not exercise a damping action on the television signal circuits. The television input signals are, of course, applied to the control grid [5 of the valve 2 whose anode circuit is coupled via the usual condenser l6 and transformer I! to the normal remaining video signal amplifier stages (not shown) of the receiver. The video signal channel through I6 and I1 is, of course, designed to handle the video signal range, e. g., plus or minus 2 megacycles as in the ordinary way.

The invention offers a further advantage as applied to receivers of the so-called single side band type, i. e., to receivers wherein the pass range is adjusted to accept only the nominal range of modulation, instead of twice that range. In a superheterodyne receiver of the single side band type the intermediate frequency pass range would be the intermediate frequency plus the modulation range or, alternatively, the intermediate frequency minus the modulation range and the local oscillator would be adjusted to bring the intermediate frequency as accurately as possible to one end of the pass range. In practice, however, the lower modulation frequencies those which differ from the carrier by about 100 k. c. or less-tend, in such a receiver, to be either accentuated by overlap of the pass band into the (undesired) side band, with consequent demodulation of both side bands, or to be attenuated by the natural attenuation of the intermediate frequency amplifier. In the interests of phase distortion such overlap should be made as small as possible, but obviously the adjustment is highly critical and in practice noticeable changes in synchronizing pulse amplitudes and in picture quality can be observed to occur as a result of small practically unavoidable drifts.

between the local oscillator and incoming carrier frequencies. By applying the present invention, which allows the relative ratio of effective synchronizing signal amplitude to picture signal amplitudes to be increased, the synchronizing signals can be maintained at a safe level. i

In order to ensure maintenance of the usually required square topped wave shape to the synchronizing pulses, the electron beam coupling or buffer circuit of this invention is preferably arranged to have a good response to harmonics of the fundamental synchronizing pulse frequency.

I claim:

1. A system for receiving television signals where a common modulated carrier is used for both the synchronizing and the picture signals comprising a picture signal intermediate frequency amplifying band pass circuit having a band width sufiiciently wide to accommodate the entire picture signals and synchronizing signal spectrum, a discharge tube including a cathode, a control electrode and an anode, condenser means for coupling the control electrode to the picture signal circuit, a second band pass circuit tuned to the said picture signal intermediate frequency coupled to the anode of said tube, said second band pass circuit having a narrow band width whereby only the low frequency components containing the synchronizing signals will be passed by said second band pass circuit.

2. A system for receiving television signals where a single modulated carrier is used for both the synchronizing and picture signals comprising a picture signal intermediate frequency amplifying band pass circuit resonant at the intermediate frequency and having a band width sufficiently wide to accommodate the entire synchronizing and picture signal spectrum, an amplifying tube including a cathode, a control electrode and an output electrode; condenser means for-impressing substantially the entire amplified frequency band across the control electrode and cathode-of said amplifier tube, a second band pass circuit coupled to the output electrode of said tube, said second band pass circuit being resonant at the said intermediate frequency and having a narrow band width capable of passing frequencies below kilocycles whereby only the synchronizing signals and the low frequency picture signals will be passed by said second band pass circuit.

DONALD LEOPOLD PLAISTOWE 

